Patterns of mangrove forest disturbance and biomass removal ...

ORIGINAL PAPER

Patterns of mangrove forest disturbance and biomassremoval due to small-scale harvesting in southwesternMadagascar

Ivan R. Scales . Daniel A. Friess

Received: 25 November 2018 / Accepted: 8 July 2019 / Published online: 18 July 2019

� The Author(s) 2019

Abstract Informal small-scale mangrove wood har-

vesting has received limited attention, though it is a

widespread threat to mangroves in many parts of the

tropics. We investigated wood use and the impacts of

harvesting on mangrove forests in the Bay of Assas-

sins in southwest Madagascar. We measured forest

structure, composition, and harvesting across 60

vegetation plots and investigated human uses of the

mangroves through Rapid Rural Appraisal techniques.

We found that unlike other mangroves in the region,

those in the Bay of Assassins are dominated by

Ceriops tagal. Tree harvesting rates are high, with a

mean of 28.7% (SD 19.4) of trees harvested per plot.

This is similar to heavily harvested mangroves in other

parts of the tropics. A comparison of tree versus

sapling importance of the different mangrove tree

species indicates that the composition of the mangrove

forest is changing, with C. tagal becoming more

important. Livelihood activities drive the harvesting of

certain species and size classes. Mangrove wood is

used mainly for the construction of traditional housing

and fencing. There are also emerging uses of man-

grove wood, including seaweed (Kappaphycus

alvarezii) aquaculture and the production of ‘sokay’,

a lime render made by burning sea shells in mangrove

wood kilns and used to improve the durability of

houses. Small-scale selective harvesting of mangrove

wood is important for local livelihoods but may have

wide-ranging impacts on forest composition and

structure. Demand for mangrove wood has grown in

relation to new commodity chains for marine products,

demonstrating the need for integrated landscape

management that considers wetland, terrestrial and

marine resources together.

Keywords Anthropogenic disturbance �Deforestation � Degradation � Forest–poverty

linkages � Provisioning ecosystem services

Introduction

Intertidal mangrove forests are globally threatened

(Polidoro et al. 2010; Hamilton and Casey 2016) by a

range of pressures such as aquaculture and agriculture,

urban development, harvesting for paper pulp, fuel-

wood, charcoal and construction materials, changes in

hydrology and sediment budgets, and sea-level rise

(UNEP 2014). There is considerable geographical

variation in the processes that lead to mangrove

deforestation and degradation, with threats differing in

both scale and type. Aquaculture has been the most

I. R. Scales (&)

St Catharine’s College, Trumpington Street,

Cambridge CB2 1RL, UK

e-mail: [email protected]

D. A. Friess

Department of Geography, National University of

Singapore, 1 Arts Link, Singapore 117570, Singapore

123

Wetlands Ecol Manage (2019) 27:609–625

https://doi.org/10.1007/s11273-019-09680-5(0123456789().,-volV)( 0123456789().,-volV)

http://orcid.org/0000-0002-8234-9251

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https://doi.org/10.1007/s11273-019-09680-5

important human activity leading to mangrove loss

globally (Thomas et al. 2017), particularly driving

mangrove change in Southeast Asia and Latin Amer-

ica (Walters et al. 2008). In sub-Saharan Africa,

harvesting for fuelwood and charcoal production are

growing threats (Feka and Ajonina 2011; UNEP 2014;

Feka 2015).

Mangrove deforestation and degradation in low-

income nations is of particular concern, as poor rural

communities rely heavily on a wide range of ecosys-

tem services provided by mangrove forests. These

include the direct provisioning of building materials,

fuelwood, charcoal, animal fodder, (shell)fish and

non-timber forest products (Bandaranayake 1998;

Dahdouh-Guebas et al. 2000; Walters 2005a; Barbier

et al. 2011; Lau and Scales 2016), alongside the

benefits that these communities gain from regulating

services (e.g., coastal protection) and cultural services

(e.g., recreation, aesthetic and spiritual values).

While communities benefit from ecosystem ser-

vices, the extraction of provisioning services can have

negative impacts on the system providing them.

Informal small-scale wood harvesting is one of the

most widespread forms of resource use in mangrove

forests (e.g. Rajkaran et al. 2004; Walters 2005a, b;

Walters et al. 2008). This is especially the case in sub-

Saharan Africa (Feka and Ajonina 2011). However,

few studies of informal mangrove harvesting have

been published. Furthermore, research on the impacts

of mangrove harvesting has mostly focused on mea-

suring mangrove cover loss through forest clearance

rather than looking at other forms of human use and

disturbance. This is in part because, unlike forest

clearance, more cryptic forms of degradation caused

by harvesting and other small-scale forest disturbances

can be difficult to detect using remote sensing methods

(Dahdouh-Guebas et al. 2005). Inaccessibility and

poor infrastructure make mangrove monitoring

through field surveys difficult and time-consuming

(Feka and Morrison 2017).

Madagascar has 2% of the total global mangrove

forest cover by area (Giri and Mulhausen 2008). With

a mean per capita daily income of $1.50, it is one of the

poorest countries in the world (World Bank 2016). As

a result, rural communities are heavily reliant on the

provisioning services of ecosystems, including man-

groves, for their basic needs (Harris 2011). This

potentially has implications for the long-term

sustainability of these ecosystems, and highlights the

importance of sustainable forest management.

We take a multidisciplinary approach to investigate

the patterns and use of small-scale harvesting on

mangrove forests in the Bay of Assassins in south-

western Madagascar, a remote region where man-

groves are used by largely subsistence-based, artisanal

fishing communities (Scales et al. 2018). First, we

characterize mangrove forest composition and struc-

ture throughout the Bay. Second, we quantify patterns

of harvesting pressure on the mangrove system and the

volume of biomass removed. Third, we investigate

household use of mangrove wood. We finish by

considering the management implications of current

mangrove uses and their impacts in the Bay of

Assassins.

Methods

Study site description

Madagascar’s mangroves covered an area of

2797 km2 in 2005 (Giri and Mulhausen 2008),

distributed primarily in bays and inlets along the

sheltered west coast. Madagascar is home to nine

species of mangrove tree (Spalding et al. 2010). A

substantial rainfall gradient exists along the western

coast, with the arid southwest of Madagascar receiving

approximately 400 mm of rainfall per year, and the

wetter northwest coast receiving approximately

2000 mm per year. This influences mangrove forest

structure along the coast, with trees of larger stature

found in mangroves in the northwest bays of Ambaro,

Ambanja and Mahajamba compared to the southwest

(Hutchison et al. 2014; Jones et al. 2016). Mangrove

biomass carbon averages 146.8 Mg C ha-1 in the

northwest (Jones et al. 2014), but only 46.2–73.9 Mg

C ha-1 in the southwest (Benson et al. 2017).

This study was conducted in the Bay of Assassins

(Helodrano Fagnemotse in Malagasy), a coastal inlet

in southwestern Madagascar (22�120S, 43�160N),

180 km north of the regional capital of Toliara

(Fig. 1). The climate is semi-arid, with approximately

400 mm rainfall per year. The inlet is fringed by c.

1300 ha of mangrove forest (Jones et al. 2016). There

are 10 villages around the bay, with a total population

of approximately 3000 (Peabody and Jones 2013).

With an average daily per capita income of less than

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610 Wetlands Ecol Manage (2019) 27:609–625

US$1.50, fishing communities in southwestern Mada-

gascar are some of the country’s most isolated and

marginalized (Harris 2011). Small-scale fishing gen-

erates over 80% of household income in these

communities and provides the majority of dietary

protein (Barnes-Mauthe et al. 2013).

Field measurements of forest characteristics

and harvested status

Our vegetation survey was based on a modified

version of a mangrove forest structure and biomass

assessment protocol set out by Kauffman and Donato

(2012). We conducted measurements of mangrove

canopy cover, species composition, forest structure,

and harvesting in 60 circular plots with a seven metre

radius (covering an area of 153.94 m2). Plot locations

were randomly selected around the Bay of Assassins

to minimize sampling biases due to edge effects or

distance from human habitation. A random number

generator was used to select distances and angles

between sampling points. This sampling design also

ensured that we randomly covered all representative

forest types across the bay.

We recorded the latitude and longitude of each plot

using a handheld Global Positioning System. The

Euclidean distance of each plot to the forest edge and

the nearest settlement were calculated in ArcGIS. We

estimated canopy cover in four cardinal directions at

the centre of each plot using a concave spherical

canopy densitometer (Lemmon 1956). In each plot we

counted and identified all trees (non-harvested and

harvested) to species level, as well as measured

diameter at breast height (DBH). We defined trees as

woody mangrove vegetation with a DBH of at least

5 cm and defined saplings as woody mangrove

vegetation with a diameter greater than 1 cm but less

than 5 cm. We counted and identified to species level

every sapling in the plot. We also counted all seedlings

within a nested circular plot of 2 m radius around each

plot centre. The DBH of live trees was measured at a

height of 1.3 m above the ground surface, or 30 cm

above the highest root if the root was higher than

1.3 m (e.g., for Rhizophora mucronata). For multi-

stemmed trees, the DBHs of all stems were measured,

though this accounted for a small percentage of the

total number of measured trees. The DBH of harvested

trees was measured either at a height of 1.3 m, or

immediately below the cut location if mangroves were

harvested at a height lower than 1.3 m. We assessed

harvested status by recording the way trees had been

harvested. Trees were recorded as either harvested at

the ground surface (1); harvested below 1.3 m height

(2); harvested above 1.3 m (3); or harvested by branch

only (4).

Calculation of statistics, forest composition

and structure indices

We calculated the tree basal area (TBA) for each tree

using the following equation: TBA (m2) = (DBH/

200)2 9 3.142 m2. For each species, we calculated:

(i) the absolute frequency (the percentage of plots in

which a species was recorded); ii) the absolute density

Fig. 1 The Bay of Assassins, southwestern Madagascar show-

ing mangrove forest cover (dark grey)

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Wetlands Ecol Manage (2019) 27:609–625 611

(the number of trees per hectare); and (iii) the absolute

cover (the total basal area of all trees per hectare). We

converted absolute measures into relative measures by

expressing the absolute measure for each species as a

percentage of the total of the absolute measures for all

trees across all species. We then calculated the

importance value for each species by adding together

relative frequency, relative density and relative cover

(Muller-Dombois et al. 1974). We calculated relative

importance values by dividing the importance values

of each species by the total importance values of all

species and multiplying by 100. We repeated this for

saplings. We calculated the regeneration rate for each

species by dividing the number of saplings by the

number of trees and multiplying by 100.

Calculations of biomass

The living aboveground biomass (AGB) and below-

ground biomass (BGB) of mangroves trees was

estimated using published, species-specific allometric

equations, in line with previous biomass studies and

accepted international protocols (e.g., Kauffman and

Donato 2012). In contrast to previous studies that

utilize a standard mangrove allometric equation (e.g.,

Abino et al. 2014; Rahman et al. 2015), we used

species-specific allometric equations (where avail-

able) to increase the robustness of our biomass

estimates. Where an equation for a particular species

was not available, it was substituted for a similar

species, e.g., Rhizophora apiculata (Putz and Chan

1986) in place of R. mucronata. In place of an equation

for Ceriops tagal we used an equation for C. australis

(Comley and McGuiness 2005); while C. australis and

C. tagal are genetically distinct species they are

botanically very similar, and until recently were

considered the same species. We also chose published

equations from locations that most closely matched

the forest characteristics and local geomorphology of

our study site where possible (Table 1). We could not

easily control for biogeographic region because of a

lack of suitable robust studies of allometry for African

mangroves. However, regional differences in allom-

etry are not generally considered significant (Chave

et al. 2014).

Surveys of mangrove wood use

To investigate mangrove wood use by local commu-

nities we used Rapid Rural Appraisal (RRA) tech-

niques (McCracken et al. 1988; Chambers 1992; Pratt

and Loizos 1992; Newing 2011). RRA covers a broad

range of methods devised to identify the problems and

strategies of households, groups and communities in a

limited time span that precludes in-depth ethnographic

fieldwork or quantitative household surveys (Pratt and

Loizos 1992; Newing 2011). The core principle of

RRA is triangulation, where data from different

techniques and informants are compared against each

other to reduce various forms of individual bias in

responses (e.g. according to age, gender or socio-

economic class) and arrive at a rigorous understanding

of major similarities and differences in resource use.

Our RRA was based on a literature review (includ-

ing data from household surveys carried out by an

environmental non-governmental organization); inter-

views with local experts working for environmental

non-environmental organizations; focus group discus-

sions in five villages; and semi-structured interviews

and transect walks with 15 key informants. We

selected these informants purposively to provide

further information on specific aspects of mangrove

use. Our informants included fishermen, individuals

involved in gleaning, and individuals involved in

mangrove wood harvesting for various purposes (e.g.

house construction and lime kiln construction). We

interviewed both men and women to ensure informa-

tion on the widest possible range of household uses

and (often gender-specific) livelihood activities relat-

ing to mangroves. Interviews lasted between 30 min

and an hour. Transect walks involved walking with

informants along a path covering major aspects of the

landscape (villages, livestock pens, farms, mangrove

forests, and dry forests) and resource uses (e.g. fishing,

gleaning, and aquaculture).

Results

The composition and structure of live mangroves

in the Bay of Assassins

While six species of mangrove tree have been

recorded in the Bay of Assassins (Benson et al.

2017), we recorded only five species in our vegetation

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surveys: Avicennia marina (afiafy in the local Mala-

gasy dialect), Bruguiera gymnorhiza (tangampoly), C.

tagal (tangambavy), R. mucronata (tangandahy), and

a single individual of Xylocarpus granatum. We also

came across a small number of Sonneratia alba trees

outside our plots. X. granatum is rarely found in

Malagasy mangroves (Gaudian et al. 1995). We

measured a total of 2667 live trees, 4861 live saplings

and 1058 seedlings across the 60 plots.

The mean tree basal area for live trees across the 60

plots was 19.37 m2 ha-1 (SD = 9.24). Our plots had a

mean canopy cover of 73.5% (SD = 23.07). Table 2

shows that the most important species in the man-

groves of the Bay of Assassins are C. tagal and R.

mucronata. C. tagal dominates, with the highest

frequency (100%), relative frequency (39.47%), rel-

ative density (59.1%), relative basal area cover

(52.17%), and relative importance value (50.25%).

Figure 2 shows that mangrove stands in the Bay of

Assassins are dominated by small trees with DBHs

smaller than 10 cm, with the diameter class distribu-

tion for all four major species in the Bay of Assassins

heavily skewed towards smaller trees.

Mangrove regeneration

We found a mean seedling density of 14,039 seedlings

ha-1 (SD = 16,089) and a live sapling density of 4554

saplings ha-1 (SD = 5901). Table 3 shows that there

is considerable variation between species in sapling

frequency, density and importance. C. tagal has the

highest sapling frequency, density and importance.

Comparing trees to saplings, the overall pattern is the

same, with C. tagal the dominant species. However,

compared to trees above 5 cm DBH, C. tagal saplings

have a higher relative importance (59.47 compared to

50.14) and R. mucronata saplings have a lower

relative importance (21.00 compared to 30.19).

Mangrove harvesting characteristics

A total of 1146 harvested trees (1241 trees ha-1) and

665 harvested saplings (720 saplings ha-1) were

sampled over the 60 plots. The mean tree harvesting

rate per plot was 28.7% (SD = 19.4), and the mean

sapling harvesting rate was 18.7% (SD = 23.1). Every

Table 1 Descriptions of

the aboveground biomass

(AGB) and belowground

biomass (BGB) allometric

equations used in this study

Species Allometric equation References

Avicennia marina AGB = 0.308DBH2.11 Comley and McGuiness (2005)

BGB = 1.28DBH1.17

Bruguiera gymnorhiza AGB = 0.186DBH 2.31 Clough and Scott (1989)

BGB = 0.199 9 q0.899 9 DBH2.22 Komiyama et al. (2008)

Ceriops tagal AGB = 0.32DBH2.056 Comley and McGuiness (2005)

BGB = 0.158DBH1.95 Comley and McGuiness (2005)

Rhizophora mucronata AGB = 0.1709DBH2.516 Putz and Chan (1986)

BGB = 0.00698DBH2.61 Ong et al. (2004)

Xylocarpus granatum AGB = 0.08233DBH2.5883 Clough and Scott (1989)

BGB = 0.199 9 q0.899 9 DBH2.22 Komiyama et al. (2008)

Table 2 Frequency, density, cover and importance of live trees of the five mangrove species recorded in sample plots

Species Absolute

frequency(% of

plots)

Relative

frequency

Absolute

density (trees

ha-1)

Relative

density

Absolute

cover

(m2 ha-1)

Relative

cover

Importance

value

Relative

importance

A. marina 13.33 5.23 75.79 2.63 1.31 6.74 14.60 4.87

B. gymnorhiza 60.00 23.53 277.16 9.62 2.03 10.49 43.64 14.55

C. tagal 100.00 39.22 1701.96 59.10 10.10 52.12 150.41 50.14

R. mucronata 80.00 31.37 825.00 28.65 5.92 30.55 90.56 30.19

X. granatum 1.67 0.65 1.08 0.04 0.02 0.10 0.80 0.27

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plot sampled contained harvested trees, with plots

ranging in harvesting rate from 1.5% to 71.4%.

Harvest pressure differed between species; C. tagal

was the most harvested species, contributing 81.2% of

all harvested trees, followed by R. mucronata (13.6%),

B. gymnorhiza (4.6%) and A. marina (0.7%). Of all C.

tagal trees measured, 35.8% were harvested, com-

pared to 16.3% of R. mucronata trees, 16% of B.

gymnorhiza trees and 9.3% of A. marina trees. The

large majority of trees (92.7%) were harvested by

cutting the tree at the trunk rather than cutting

branches.

The mean DBH of harvested trees was 9.21 cm

(SD = 3.67). Looking at all species together, the most

harvested size class was 5–9 cm DBH. These

accounted for 61.59% of all harvested stems. Table 4

compares the mean DBH of non-harvested and

harvested trees for each species. It shows that for

every species apart from A. marina, the mean DBH of

harvested species was significantly greater

Fig. 2 Frequency distributions of the DBH (cm) of non-harvested and harvested trees of the four major species found in the Bay of

Assassins

Table 3 Live sapling frequency, density and regeneration rate for each species

Species Absolute

frequency (%

of plots)

Relative

frequency

Absolute

density

(saplings ha-1)

Relative

density

Importance (relative

frequency ? relative

density)

Relative

importance

Regeneration

rate

A. marina 3.33 1.52 12.99 0.29 1.81 0.90 13.95

B. gymnorhiza 65.00 29.55 350.79 7.72 37.27 18.63 102.76

C. tagal 85.00 38.64 3647.53 80.31 118.95 59.47 153.70

R. mucronata 66.70 30.30 530.51 11.68 41.98 21.00 124.76

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(p =\ 0.05) than the mean DBH of non-harvested

trees, indicating that harvesters are preferentially

selecting trees that are larger than the average size.

The AGB for all our plots averaged 161.9 Mg ha-1

(SD = 68.6 Mg ha-1). However, harvesting pressure

means that this average is below the potential AGB

that is available at this site. A mean AGB of 46.7

Mg ha-1 was removed through harvesting, though a

large standard deviation (42.4 Mg ha-1) is reflective

of the substantial variation in total harvested AGB

across the 60 plots, ranging from 0 to 163.3 Mg ha-1.

This means that the site AGB is 22.4% lower than the

potential AGB available at this site.

Ceriops tagal was the preferred species to be

harvested, accounting for 72.9% of the total AGB of

harvested wood (Table 5). This was followed by R.

mucronata (20.3% of all harvested wood). Size classes

were also differentially harvested. Trees with a DBH

between 5.0 cm and 9.9 cm accounted for an average

of 27.9% of all harvested trees. This is a dispropor-

tionate contribution, considering the small individual

volume of trees in this smaller size class, compared to

the larger trees harvested. Similar to the overall

volumes, C. tagal was strongly preferred as a smaller

pole, alone accounting for 21.4% of all AGB har-

vested. We discuss the uses of the different species and

size classes in ‘‘Human uses of harvested mangrove

wood’’ section.

Spatial patterns of mangrove harvesting

The plots we sampled were a mean distance of 1159 m

(SD = 606) from the nearest settlement, with a

minimum of 233 m and a maximum of 2801 m. Plots

were a mean distance of 92 m from the nearest

mangrove edge (SD = 62), with a minimum of 15 m

and a maximum of 352 m. Table 6 shows the results of

correlations conducted between distances of the plots

from the nearest human settlement; distances of the

plots from the nearest mangrove edge; and various

measures of mangrove harvesting.

The results show that there are significant inverse

correlations (p =\ 0.05) between the distance from

the nearest human settlement and the percentage of

trees harvested; the percentage of 5–9 cm and

10–14 cm DBH trees harvested; and the percentage

of B. gymnorhiza and C. tagal harvested. However,

there was no significant correlation between distance

from the nearest human settlement and the percentage

of A. marina or R. mucronata harvested. None of the

correlations between distance from the nearest man-

grove edge and tree harvesting were significant. These

results show that mangrove forests are more heavily

harvested closer to human settlements but that

distance from the mangrove edge (and thus access

from the sea or land) does not seem to influence

harvesting pressure.

Table 4 Comparison of the mean DBH in cm (with standard deviations) of non-harvested and harvested trees

Mean DBH (cm) of non-harvested trees Mean DBH (cm) of harvested trees t p

All trees 8.53 (3.55) 9.21 (3.67) - 5.26 \ 0.001

A. marina 12.48 (8.03) 10.66 (3.53) 1.16 0.26

B. gymnorhiza 9.20 (3.98) 11.81 (4.64) - 4.38 \ 0.001

C. tagal 8.16 (2.97) 8.96 (3.62) - 5.70 \ 0.001

R. mucronata 8.85 (3.59) 9.74 (3.21) - 3.09 0.002

Table 5 Percentage of

above-ground biomass

(AGB, Mg Ha-1) harvested

by species and size class.

Total = 100%

DBH[ 5.0\ 10.0 cm (%) DBH[ 10 cm (%)

A. marina 0.20 0.58

B. gymnorhiza 0.56 5.21

C. tagal 21.40 51.52

R. mucronata 5.77 14.53

A. marina 0.00 0.23

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Human uses of harvested mangrove wood

The Rapid Rural Appraisal showed that mangrove

trees in the Bay of Assassins were harvested: (i) as

fuelwood for domestic cooking; (ii) as a building

material for housing and fencing; (iii) as a building

material for seaweed aquaculture; and (iv) for the

construction of lime kilns. Respondents suggested that

the use of mangrove wood in lime kilns and in seaweed

aquaculture are recent, having only developed over the

last 10 years. Mangrove wood was also recently used

in sea cucumber (Holothuria scabra) aquaculture to

construct growing-out enclosures. However, man-

grove wood has now been replaced with stronger

and more durable steel rods for this use.

With regards to domestic fuelwood, saplings and

trees less than 10 cm DBH were sometimes used but

wood from neighbouring terrestrial dry forests was

preferentially used as a fuel as it is usually drier and

more easily combustible. The most frequent use of

mangrove wood was for house and fence construction.

These activities almost exclusively use mangrove

wood rather than wood from terrestrial forests. Larger

poles (between 10 cm and 20 cm diameter) were used

to build the frames of houses (Fig. 3a), with smaller

poles (5 to 10 cm diameter) used to support wall and

roof material. Respondents generally favoured R.

mucronata for the construction of house frames

because of its resistance to decay.

Poles of C. tagal were used in the construction of

fencing around houses (background of Fig. 3b) and to

construct livestock enclosures. Fences are particularly

important due to high incidences of banditry in the

region. Fence poles were used in large quantities due

to frequent maintenance and replacement require-

ments, with poles lasting 2 or 3 years. Our informants

told us that fence construction was the most common

reason for harvesting mangrove wood. This is sup-

ported by the vegetation survey (‘‘Mangrove harvest-

ing characteristics’’ section), which showed that a

disproportionate volume of harvested AGB was C.

tagal stems with a DBH of between 5 and 9.9 cm (the

preferred species and size class for fencing).

A more recent use of mangrove wood in the Bay of

Assassins involves seaweed (Kappaphycus alvarezii)

aquaculture. Saplings and trees with a DBH of 5 to

9 cm and a length of approximately 1 m were used as

stakes for seaweed farming. The stakes provide anchor

points for ropes on which the algae grow, but needed

replacing every 6–24 months. Mangrove poles were

also used to build tables to dry the seaweed (Fig. 3b).

Another more recent use of mangrove wood in the

Bay of Assassins involved the construction of lime

kilns (Fig. 3c). These were used to produce ‘sokay’, a

sea-shell based lime render used to improve the

durability of house walls (Fig. 3d). To construct a kiln,

layers of sea shells are sandwiched between layers of

mangrove wood. Burning the shells converts calcium

Table 6 Correlation coefficients for relationship between distance from the nearest human settlement and distance from the nearest

mangrove edge

Distance from nearest human settlement

(m)

Distance from nearest mangrove edge

(m)

r p r p

Percentage of trees harvested - 0.43 \ 0.001 - 0.21 0.10

Percentage of saplings harvested 0.08 0.53 - 0.03 0.80

Percentage of trees 5–9 cm DBH harvested - 0.42 \ 0.001 - 0.23 0.07

Percentage of trees 10–14 cm DBH harvested - 0.39 0.002 - 0.12 0.37

Percentage of trees 15–19 cm DBH harvested - 0.13 0.36 0.18 0.22

Percentage of trees C 20 cm DBH harvested - 0.09 0.71 0.36 0.10

Percentage of A. marina trees harvested - 0.40 0.37 - 0.12 0.80

Percentage of B. gymnorhiza trees harvested - 0.38 0.02 - 0.09 0.56

Percentage of C. tagal trees harvested - 0.47 \ 0.001 - 0.29 0.03

Percentage of R. mucronata trees harvested - 0.23 0.11 - 0.18 0.21

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carbonate (CaCO3) to calcium oxide (CaO). Water is

added to the resulting powder and the paste is then

applied to house walls. Lime render was the preferred

wall material because of its relative durability com-

pared to the mangrove saplings and reeds used in

traditional houses (Fig. 3b). Madagascar regularly

experiences tropical cyclones with strong winds and

heavy rain, and respondents reported that lime

rendered walls are more resistant to cyclone damage.

We found, based on measurements of 10 kilns

found in five villages, that a typical lime kiln measures

c. 2.6 m in length, 2.3 m in width and 1.2 m in height,

using mangrove poles of c. 10–15 cm diameter. Kiln

construction requires c. 120 poles of mangrove wood,

with a total volume of c. 2.4 m3. Respondents stated

that their preferred species of mangrove wood for kiln

construction was R. mucronata because it burns hotter

than other mangrove and terrestrial forest tree species,

Fig. 3 Examples of the use of mangrove wood: saplings and

trees used in house construction (a); saplings used as fencing

(background of b); trees used to construct seaweed drying racks

(foreground of b); and large trees used to construct kilns

(c) which produce lime render for houses (d)

123


producing a purer lime powder. However, in our

village surveys and observations of the kilns we noted

that other species, particularly C. tagal and B.

gymnorhiza were also used, presumably due to

availability. Kiln builders stated that the ideal size

class for mangrove trees to build kilns was between

10 cm and 15 cm DBH. Trees over 20 cm DBH were

too large for an individual to carry out of the

mangroves, and as such are harvested infrequently.

Discussion

Forest structure, composition and regeneration

The mix of species in the Bay of Assassins is typical of

southwest Madagascar’s mangroves, with five species

recorded in our plots, compared to a total of six species

known to be present in the Bay (Benson et al. 2017).

However, in comparison to other sites in southwest

Madagascar, the mangroves of the Bay of Assassins

are unusual in that they are strongly dominated by C.

tagal. In our survey this species was found in all plots

and was the dominant species in 65% of plots,

accounting for 65.52% of all trees sampled. In

comparison, the mangroves of the Mangoky River

Delta, located approximately 80 km north of the Bay

of Assassins, are dominated by R. mucronata and A.

marina (Rakotomavo and Fromard 2010), as are the

mangroves of the Tulear Lagoon, 180 km to the south

of our study site (Laroche et al. 1997).

It is possible that geomorphology plays a role in the

differences between these different sites, since man-

grove species distribution is heavily influenced by

abiotic factors such as inundation frequency (Leong

et al. 2018). However, information on the geomorphic

setting of these locations is not available. The

dominance of C. tagal in the Bay of Assassins may

also be driven by historical harvesting pressure on R.

mucronata, coupled with C. tagal being the dominant

regeneration species (as suggested by its higher

importance values). Other East African case studies

have shown that the preferential harvesting of R.

mucronata (due to its high density and calorific value)

leads to a change in mangrove community dominated

by C. tagal over time (Kairo et al. 2002). However, a

lack of historical data on harvesting or vegetation

structure at this sites means we are unable to test this

hypothesis in the Bay of Assassins.

We also found that for all four mangrove tree

species commonly found in the Bay of Assassins (A.

marina, B. gymnorhiza, C. tagal, and R. mucronata),

mangrove forests were dominated by small trees that

were \ 10 cm DBH. This could be due to a combi-

nation of climatic pressures and harvesting pressure.

Mangrove growth rates and biomass accumulation are

often linked to climatic variables such as cyclone

frequency (Simard et al. 2019) and, more importantly,

long term distribution of rainfall (Krauss et al. 2007;

Sanders et al. 2016). Arid conditions (such as those in

our study site) are associated with high soil salinities,

which may restrict mangrove growth (Cintron et al.

1978) and cause them to allocate greater biomass to

the below-ground fraction to minimize salinity gradi-

ents around the roots and aid in water uptake (Clough

et al. 1997; Alongi et al. 2005). This would lead to

lower biomass partitioning (and DBH) above ground.

Extreme aridity has thus been suggested as a reason for

low biomass and small DBH for mangroves along the

arid coast of Western Australia (Clough et al. 1997),

though other studies in Australia show different results

(Comley and McGuiness 2005). Thus, harvesting

pressure probably also plays a role in controlling size

classes. In general, harvested trees were significantly

larger than non-harvested trees (with the exception of

A. marina, which is not preferred as harvested material

to the same degree as the species of Rhizophoraceae),

with the mean DBH of remaining non-harvested trees

being an average of\ 9 cm.

Harvesting pressure and patterns in the Bay

of Assassins

Unlike many other parts of Madagascar (cf. Jones et al.

2014; Giri and Mulhausen 2008), the clear cutting of

mangroves is, at present, relatively rare in the Bay of

Assassins. The region experienced a rate of mangrove

cover loss of 0.27% per year between 2002 and 2014

(Benson et al. 2017), compared to 1.19% per year for

mangrove in northwestern Madagascar between 1990

and 2010 (Jones et al. 2014); 0.85% per year between

1951 and 2000 in the Mangoky River delta (Rako-

tomavo and Fromard 2010); and 0.52% per year for the

whole of Madagascar between 2000 and 2005 (Giri

and Mulhausen 2008). Thus, mangrove forests in the

Bay of Assassins are not, at present, facing the large-

scale deforestation threats that other mangrove forests

in Madagascar are experiencing. In other parts of

123


Madagascar, mangrove deforestation has been driven

primarily by clearance for agriculture, large-scale

commercial shrimp aquaculture, and commercial

charcoal production (Giri and Mulhausen 2008;

Rakotomavo and Fromard 2010; Jones et al. 2016),

none of which are currently resource uses found in the

Bay of Assassins. This is possibly because the Bay of

Assassins is too arid and currently too inaccessible

from major markets for these alternative land uses to

be financially viable.

During the vegetation survey we only came across

one instance of clear cutting, which was outside our

randomly-assigned vegetation plots and was related to

a large order for lime render. Instead, the ecological

impacts of mangrove wood harvesting are more

cryptic (Dahdouh-Guebas et al. 2005). We did not

come across a single unharvested plot during our

extensive survey. Thus, the main stressor on the Bay of

Assassins system is selective harvesting for a range of

household construction activities, particularly fencing.

However, even with harvesting, mean canopy cover

was high (73.5%, SD = 23.07).

Comparison with the small number of mangrove

harvesting studies conducted across the tropics

(Table 7) suggests that mangrove harvesting pressure

is high in the Bay of Assassins. With a mean of 28.7%

of trees harvested, the magnitude is similar to the

figure of 31.7% reported for a heavily harvested site in

the Philippines (Walters 2005b). In a study of various

sites in Micronesia, the overall harvest pressure was

found to be 10%, with a removal of mangrove wood

equivalent to 11% of the standing volume (Hauff et al.

2006).

All four of the major mangrove species were

harvested. The principle method of harvesting,

whereby trees are cut at the base of the trunk, has

important management and sustainability implica-

tions. For both C. tagal and R. mucronata cutting in

this way kills the tree, since members of the

Rhizophora and Ceriops genera lack reserve meris-

tems and do not coppice (Hamilton and Snedaker

1984). This means that for the two dominant and most

harvested species in the mangroves of the Bay of

Assassins regeneration is entirely dependent on

replacement by new seedlings.

We found that for all species apart for A. marina,

the mean DBH of harvested trees was significantly

greater than that of non-harvested trees, indicating a

preferential selection of larger than average trees. The

results of our Rapid Rural Appraisal indicate a

preference for large ([ 10 cm DBH) R. mucronata

trees, particularly for the construction of house frames

and in the production of lime render. There was a

correlation observed between distance from the near-

est settlement and the percentage of C. tagal trees

harvested, because this is a common species and can

be harvested in many locations. In contrast, no such

correlation was observed for R. mucronata trees. Our

Table 7 Comparison of selective harvesting pressures in mangroves across the tropics

Harvesting

pressure (%)

Species preferred Use(s) Location References

7.7–33.4 Rhizophora mangle, Laguncularia

racemosa

Construction Rıo Limon,

Venezuela

Lopez-Hoffman et al.

(2006)

10 Rhizophora apiculata (fuelwood),

Bruguiera gymnorhiza (fuelwood,

construction)

Fuelwood, construction

materials

Kosrae,

Micronesia

Hauff et al. (2006)

28.7

(SD = 19.3)

Ceriops tagal, Rhizophora mucronata Construction materials Bay of

Assassins,

Madagascar

This study

30.5–44.9 Avicennia marina, Ceriops tagal Household use Metinaro,

Timor Leste

Calculated from Alongi

and de Carvalho

(2008)

31.7 Rhizophora spp. (for construction) Fuelwood, house

construction, fence

construction

Visayas,

Philippines

Walters (2005b)

123


key informants informed us that as large ([ 10 cm

DBH) R. mucronata trees are particularly desirable,

resource users will travel further to extract them.

Taken together with the importance values of trees

and saplings, these results suggest a shift in commu-

nity composition and structure, with harvesting lead-

ing to a relative absence of large trees, and an increase

in the importance of younger and smaller C. tagal

trees. This is similar to other studies that have shown

that harvesting pressure on Rhizophora spp. is not

driven by distance; instead, resource users will travel

to locations depending on resource availability (Dah-

douh-Guebas et al. 2000; Palacios and Cantera 2017).

In South Africa, a study found that R. mucronata was

heavily harvested while adjacent species such as A.

marina were not (Rajkaran et al. 2004).

Rhizophora species have historically been pre-

ferred for harvest due to their resistance to insect pests

when used as construction materials (e.g. De Puydt

1868) and their high calorific value, which makes

them ideal fuelwood for domestic purposes and, in the

case of the Bay of Assassins, in the construction of

lime kilns. They are still preferred by local commu-

nities for these reasons today (e.g. Dahdouh-Guebas

et al. 2000). This fact has important implications for

the spatial planning of potential management areas

versus the spatial distribution of preferred species.

Contributions of mangrove harvesting to local

livelihoods

Mangroves are often used for fuelwood and charcoal

production in sub-Saharan Africa (Dahdouh-Guebas

et al. 2000; Rajkaran et al. 2004) and other tropical

coastal areas (e.g. Oo 2002; Sudtongkong and Webb

2008; Moriizumi et al. 2010), though this was not the

case in the Bay of Assassins. Forest users instead

prefer to use fuelwood resources collected from

terrestrial spiny forest, which is itself heavily threat-

ened (Seddon et al. 2000; Casse et al. 2004).

In the Bay of Assassins, mangrove wood is used

primarily in the construction of houses and fences,

where its ability to resist decay makes it a favoured

material. Mangrove wood plays an important role in

household security, as it is used to build fences to

protect property (including livestock) from banditry. It

is also used in the reinforcement of house walls (and

thus protection from cyclones) through the production

of lime render. The harvesting of 5–9 cm trees, which

are primarily used as fencing material, accounts for the

majority of above-ground mangrove wood biomass

removed.

We have also documented emerging uses of

mangrove wood. The recent development of seaweed

and sea cucumber aquaculture has created new

demand for mangrove wood. In addition, the produc-

tion of lime render using mangrove wood kilns is also

creating demand for mangrove wood. Lime render has

only recently been identified as a significant resource

use associated with mangrove harvesting in the Bay of

Assassins (Scales et al. 2018) and, to our knowledge,

has not been recorded elsewhere in Madagascar or

sub-Saharan Africa. Rendering the walls of an average

house requires the lime produced by two kilns, which

themselves require 4.8 m3 of mangrove wood. To put

this in perspective, the frame and un-rendered walls of

a typical house in the region require 0.9 m3 of wood

(Rasolofo 1997). A rendered house uses approxi-

mately six times more mangrove wood than an un-

rendered house.

Reconciling the ecology and management

of the Bay of Assassins

This study highlights the strong dependence that local

communities have on the mangrove forests in the Bay

of Assassins, with provisioning ecosystem services

such as seafood (Aina 2010) and wood products

(Scales et al. 2018; this study) making an important

contribution to livelihoods and the local economy. It is

crucial to manage these resources sustainably, so that

they can continue to provide wood and fuel resources

into the future without adverse ecological impact.

How sustainable are current mangrove harvesting

practices? Extrapolating our data on harvested trees

from the 60 vegetation plots (0.92 ha) to the 1300 ha

of mangroves in the Bay of Assassins indicates that

there are approximately 2.76 million live trees in the

Bay of Assassins with a DBH of 5–9 cm. A household

survey carried out in seven of the ten villages in 2014

estimated that mangrove harvesting for fence poles

accounted for approximately 50 stems per person per

year (Blue Ventures 2015). This translates a total of

150,000 stems per year, or 5.43% of the trees in the

5–9 cm size class, for all the villages in the Bay of

Assassins.

Our study suggests that while mean canopy cover

(73.5%) remains high and the mangroves of the Bay of

123


Assassins are currently regenerating, ecological

impact is apparent. Looking to the future, it is likely

that human pressures on the mangroves of the Bay of

Assassins will continue to increase. The human

population in the region is growing. Data are scarce,

but informants reported significant in-migration over

the last 5 years. This is supported by Epps (2007), who

found that\ 30% of inhabitants in Lamboara and

Ampasilava were born in those settlements. Coastal

areas in southwest Madagascar are experiencing rapid

in-migration, as poor agricultural households move to

the coast seeking more secure livelihoods (Brugge-

mann et al. 2012). A growing population means more

demand for mangrove wood, especially for housing

and fencing. The harvesting of 5–9 cm trees, which

are primarily used as fencing material, accounts for a

disproportionate percentage of above-ground man-

grove wood biomass removed, considering their small

individual volume, and those managing the mangroves

of the Bay of Assassins may need to look for

alternative fencing material if demand continues to

grow.

In addition to population growth, the region is also

experiencing economic change, which is increasing

demand for mangrove wood. Over the last 10 years the

region has become connected to global commodity

chains of sea cucumber, seaweed and octopus (Aina

2010; Barnes-Mauthe et al. 2013). This has created

new pressures on mangroves through the wood needed

for aquaculture, for example to construct sea cucum-

ber enclosures (although these are now made of steel

rods), seaweed anchors and seaweed drying tables.

Growing income from aquaculture has also led to

increased demand for lime render for houses, as

households who are able to benefit from the new

commodity chains for marine products choose to

improve the durability of their houses.

The harvesting of sea cucumber, seaweed and

octopus have all increased over the last 10 years (Aina

2010; Barnes-Mauthe et al. 2013; Blue Ventures

2015). For example, the harvest of K. alvarezii in 2014

was over 55 tonnes, three times that of 2012. This has

important implications for the regions mangroves as

three tonnes of mangrove wood are required to

produce one tonne of dried seaweed (Blue Ventures

2015). Key informants also reported an increase in the

use of lime render. This is supported by previous

household surveys. In 2006, 28% of buildings in

Lamboara village had walls covered in lime render

(Epps 2007). By 2014 this figure had gone up to 65%

(Blue Ventures 2015). Lime production is a time-

consuming process, requiring men to find, cut, and

carry heavy trees. Most households do not have the

time to render their own walls, focusing their attention

on fishing and gleaning. However, with a rise in

income from aquaculture, wealthier households are

paying others to produce lime. As a result, lime render

is considered a status symbol. Demand for lime render,

and thus for mangrove wood to use in lime kilns, is

likely to grow with a growing population and/or rising

incomes. Growing demand for large ([ 10 cm DBH)

R. mucronata trees is occurring in a mangrove forest

that is dominated by C. tagal trees below 10 cm DBH.

To date, efforts to manage natural resources in the

Bay of Assassins have tended to focus on a single

ecosystem or species, for example establishing closed

seasons for octopus fisheries, introducing new tech-

niques for sea cucumber aquaculture, and creating

protected areas for mangrove forest where wood

harvesting is forbidden (Andriamalala and Gardner

2010; Aina 2010; Benbow et al. 2014; Cripps and

Gardner 2016). The key resource management lesson

is that livelihoods in coastal communities cut across

terrestrial, wetland and aquatic ecosystems. This

means that mangroves are socio-ecologically linked

to other ecosystems and must therefore be managed as

part of broader landscape-based approaches. For

example, increases in wealth from marine resources

such as sea cucumber, algae and octopus are likely to

lead to greater demand for more durable housing

material which, unless alternatives are found, will lead

to increased demand for lime render and thus place

pressure on mangrove forests. Another potential cross-

ecosystem linkage involves domestic fuelwood. While

mangrove wood is the preferred material for construc-

tion, terrestrial dry forests are the preferred source of

fuelwood. However, a reduction in terrestrial dry

forest (for example due to forest clearance for

agriculture or over-harvesting of fuelwood), is likely

to lead to more pressure on mangroves as households

seek out alternative sources of domestic fuel. There-

fore, management policies need to carefully model

expected increases in demand for mangrove wood for

different uses and consider developing alternative

sources of domestic fuel and alternative ways to

improve the durability of houses.

123


Conclusions

Small-scale wood harvesting by local communities

can be a significant driver of degradation in many

mangrove forests across the tropics (UNEP 2014).

Given the importance of mangroves to the livelihoods

of coastal communities in the tropics and the potential

impact of human activities on mangrove cover,

structure and composition, it is remarkable that so

few studies of small-scale mangrove harvesting have

been published. Our understanding of mangrove uses

and their ecological impacts is hindered by a narrow

focus on land cover change measured through remote

sensing, which misses more cryptic forms of ecolog-

ical change (Dahdouh-Guebas et al. 2005). There is

thus a need for field surveys, combined with studies of

human resource use, to reveal cryptic processes of

ecological change that are not easily revealed through

remote sensing.

This study, based on a combination of mangrove

vegetation surveys and RRA techniques, has shown

that poor coastal communities in southwest Madagas-

car use mangrove wood for both domestic and

commercial purposes. We have shown high harvesting

pressure. Our study also shows that harvesting is

spatially variable and can have observable ecological

impacts on multiple mangrove forest community

characteristics. It also shows that the local community

has clear intended uses, which drives the harvesting of

particular species and size classes.

Environmental policy in the tropics often focuses

on demographic change and the implications of

population growth on resource use. There is a lack

of understanding of the diverse ways in which low-

income households, particularly in remote regions,

rely on mangrove resources and a tendency to assume

all poor households use natural resources in the same

way (Angelsen and Wunder 2003; Belcher 2005). This

study found that resource extraction, poverty and

livelihoods are interlinked, with increasing livelihood

status changing the type of resource extracted (in this

case R. mucronata wood for lime kiln construction).

Thus, livelihood status is likely to impact the species

preferred and the volume harvested. Even remote

coastal regions such as the Bay of Assassins are

increasingly linked to global commodity chains,

leading to changes in resource use. More attention

needs to be paid, through household surveys, to the

quantities of mangrove wood used for different small-

scale purposes, and how these vary according to socio-

economic characteristics such as wealth and migration

status, so that adverse ecological impacts on forest

resources can be better quantified, anticipated and

managed.

Finally, our study suggests that mangroves must be

managed as part of broader landscape-based

approaches. In the Bay of Assassins resource man-

agement policies have tended to focus on single

species and ecosystems. For example, there have been

efforts to improve the sustainability of marine

resource use through the introduction of a closed

season for octopus fisheries and new techniques for sea

cucumber aquaculture. However, income from marine

resources such as sea cucumber, seaweed and octopus

has led to growing demand for lime render to improve

the durability of houses. Lime render is made in

mangrove wood kilns and growing demand has led to

increased pressure on mangrove forests. In turn, the

loss of mangroves has implications for natural coastal

fisheries, due to their role in the life-cycle of fish and

other marine fauna (Whitfield 2017) that also play an

important role in local livelihoods. Therefore, man-

agement plans must carefully consider how changes in

the socio-ecological dynamics of one natural resource

are likely to impact those of other resources that

households depend on. Research has shown that

governance decentralization and community man-

grove management can improve mangrove condition

if strong community institutions are present to

enforcement common rules of natural resource man-

agement (Primavera 2001; Sudtongkong and Webb

2008; Damastuti and de Groot 2017). Community-

based management approaches will therefore be of

crucial importance in any attempt to manage man-

groves as part of a broader landscape-based approach.

Acknowledgements This research was funded through an

Environment and Sustainability Grant from the Royal

Geographical Society (with IBG). We thank Blue Ventures for

their logistical support; Michael Strongoff, Jose Njara

Ranaivoson and Helen Scales for fieldwork assistance; and

Philip Stickler (Department of Geography, University of

Cambridge) for cartographic assistance. We also thank the

two anonymous reviewers for their comments and suggestions,

which helped to improve the clarity of the paper.

Open Access This article is distributed under the terms of the

Creative Commons Attribution 4.0 International License (http://

creativecommons.org/licenses/by/4.0/), which permits unre-

stricted use, distribution, and reproduction in any medium,

provided you give appropriate credit to the original

123


http://creativecommons.org/licenses/by/4.0/

http://creativecommons.org/licenses/by/4.0/

author(s) and the source, provide a link to the Creative Com-

mons license, and indicate if changes were made.

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